Methods of depositing nickel-iron films



Unite States Patent Office 3,027,309 Patented Mar. 27, 1962 3,027,309METHODS OF DEPOSKTHNG NICKEL-IRON FEMS James Henry Stephen, Abingdon,England, assignor to United Kingdom Atomic Energy Authority, London,England No Drawing. Filed Oct. 5, 1959, Ser. No. 844,205 Claimspriority, application Great Britain Oct. 9, 1958 5 Claims. (Cl. 204-43)This invention relates to methods of depositiing nickel/ iron films andrelates particularly to a process for electrodepositiing approximately80/20 nickel/iron thin films having substantially rectangular hysteresisloop onto cylindrical formers from an aqueous solution of nickel andferrous sulphamates.

Thin films of approximately 80/20 nickel/iron composition have beendeposited by various methods for use in magnetic switching and memorydevices, the film thickness varying from 1000 A. to over 10,000 A.depending on the application. The films can be made to have an easydirection of magnetisation which is established by carrying out thedeposition in a magnetic field which aligns the film as it is deposited.Films so produced have hysteresis loops in the direction of easymagnetisation which are rectangular with a high degree of remanence,making them very suitable for storing information.

Methods used for depositing such films have included evaporation,chemical deposition, and electrodeposition. In the case ofelectrodeposition, an equeous solution of mixed nickel and ferroussulphates has been used as the electrolyte. Unfortunately filmsdeposited from sulphate solutions are in a highly stressed (tensile)condition, which affects the magnetic characteristics and may cause thefilm to peel from the substrate. Iron is in fact normally removed fromconventional nickel sulphate plating baths because its presence adds tothe stress in the plated film.

The present invention provides a process for electrodepositiing 80/20nickel/iron films having the required magnetic characteristics in arelatively unstressed condition, and comprises electrodepositing from anaqueous solution of mixed nickel and ferrous sulphamates under carefullycontrolled conditions.

According to the present invention, in a process for electro-depositingan approximately 80/20 nickel/iron thin film having a substnatiallyrectangular hysteresis loop on to a former in an aligning magneticfield, the former is made the cathode in an electrolytic cell containinga solution of nickel and ferrous sulphamates, the concentration ofnickel ions in the solution is made greater than 100 gm./ litre, theratio of nickel ion concentration to ferrous ion concentration is madebetween 35:1 and 40:1, the pH is made between 2 and 3.5, the solutiontemperature is maintained below 30 C., the deposition current isregulated such that the deposition potential lies above 890 rnv., andthe solution concentration is maintained substantially constant over thesurface of the cathode.

The solution temperature is preferably maintained in the range 18 to 22C., the deposition current being regulated such that the depositionpotential is between 920 and 950 mv.

Preferably the cathode surface is initially struck by passing adeposition current sufficiently large to deposit a nickel foundation onthe cathode.

With the cylindrical former the solution concentration may be maintainedsubstantially constant over the surface of the cathode by rotating thecathode on its own axis aligned substantially vertically and releasing astream of gas bubbles below the cathode.

Also according to the present invention an apparatus for use in aprocess as aforesaid comprises a vessel for containing an electrolyte,means for suspending a cylindrical cathode vertically in the electrolyteadapted to rotate said cathode on its own axis, and a duct having anorifice for releasing a stream of gas bubbles below said cathode.

Also according to the present invention an electrolyte for use in aprocess as aforesaid comprises an aqueous so lution of nickel andferrous sulphamates where the nickel ion concentration is greater thangrn./litre, the ratio of nickel ion concentration to ferrous ionconcentration is between 35 :1 and 40: 1, and the pH is between 2 and3.5.

By Way of example the deposition of a 1.6 micron (16,000 A.) film onto aformer consisting of a copper rod 4 cm. long and 0.228 cm. in diameterwill now be described.

The surface of the rod is prepared for deposition by degreasing inconcentrated sulphuric acid and etching for 30 seconds in a nitric acidpolishing bath. The rod is then electropolished for 15 minutes in anorthophosphoric acid bath and the polished rod washed four times withdistilled water. The polished surface must support a water film and isnot allowed to dry before inserting in the electrolyte. Polyethylenemasks may be slipped onto the rod after polishing if it is desired. torestrict the length of the magnetic film.

The electrolytic cell is a cylindrical glass container 6.5 cm. indiameter and 12 cm. high. The anode, of pure nickel sheet, iscylindrical and fits closely the inner surface of the container. Thepolished rod forms the cathode and is suspended vertically in the centreof the cell from the shaft of an electric motor which rotates at onerevolution per second. At the bottom of the cell immediately below thecathode is the orifice of a small capillary tube through which nitrogengas (oxygen-free) is blown at 10 lbs/sq. in. to produce a stream ofbubbles which rise past the rod.

To provide an aligning magnetic field along the length of the rod, al6-turn coil of copper tube is wound round the outside of the glasscontainer. The coil is excited at 50 c./s. with a current of 100 a.(R.M.S.) to produce a peak field of 250 gauss. The coil is cooled bypassing water through it.

The electrolyte consists of an aqueous solution of nickel sulphamate(Ni(NH SO and ferrous sulphamate (Fe(NH SO plus boric acid (H BO Boricacid in concentrations approaching saturation is a normal constituent ofnickel plating solutions.

The nickel ion concentration is 113 gm./ litre, the ferrous ionconcentration is 2.82 gm./litre:, and the boric acid concentration is 30gm./litre. The pH of the solution is adjusted to 2.5 by adding sulphamicacid (H(NH SO The pH must be adjusted during the life of the solution asmust the nickel and iron concentrations.

The electrolyte temperature is 18-22 0, ie room temperature. Thepotential which exists between the cathode and the solution duringdeposition depends on the deposition current and is measured using asaturated calomel reference electrode. The electrode is connected to thecell via a salt bridge comprising a first tube containing KCl solutionconnected between the calomel electrode and one arm of a two-waystop-cock, and a second tube containing nickel sulphamate solutionconnected between the other arm of the stop-cock and the cell. In thisway the KCl and the nickel sulphamate solutions only make contact in athin film at the stop-cock, and contamination of the electrolyte bychloride, which would lead to the deposition of stressed films, isprevented. The end of the salt bridge in the cell is placed as close tothe cathode as possible without masking it.

The deposition current is obtained from a constant-current electroniccircuit, the value of the current being controlled by feedback from themeasured deposition potential in a sense to keep the latter constant.

The actual deposition is carried out as follows. Before the cathode isplaced in the electrolytic solution the current source is set to supply0.5 ma, which flows immediately the cathode is immersed and so preventsthe newly polished copper surface from dissolving. The magnetic field,the nitrogen bubbler and the motor are set into operation.

A current of 150 ma. is first passed for approximately seconds to strikethe cathode surface. This has the effect of raising the depositionpotential and so depositing a layer of almost pure nickel on the copper.This layer is sufficiently thin not to affect the magnetic properties ofthe final film, but provides a uniform foundation for the film. Thecurrent is then reduced to ma. (current density approximately 10 ma./sq. cm. of cathode area), at which current the deposition potential is950il0 mv. When a total charge of 16 coulombs has passed, as measured bya coulombmeter, the current is reduced to 0.5 ma. and the cathode, nowcoated with a nickel/iron film approximately 1.6 microns thick, isremoved from the electrolyte. After washing first in distilled water andthen in alcohol (iso-propyl and ethyl have been used) the rod is driedunder an infra-red lamp.

For thicker or thinner films the total charge passed is variedaccordingly.

Films deposited by the above- 'escribed process have rectangularhysteresis loops with high remanence and a coercive force of about 3:1oersteds.

The magnetic aligning field may be either AC. or DO. The alignment canbe other than axial as in the described example; for example a currentpassed along the axis of the rod will produce circumferential magneticalignment, and a combination of axial and circumferential fields willproduce helical alignment.

The conditions under which deposition takes place have been found to berather critical if satisfactory films are to be produced:

(a) The ratio of the concentration of nickel ions to ferrous ions in thesolution must be between :1 and :1. Above and below this range thedeposited alloy becomes nickel-rich and iron-rich respectively.

(b) The concentration of nickel ions must be greater than 100 gm./litreto keep the deposition potential sufficiently low.

(0) As is well known in the electrochemical art, the composition of thedeposited film depends on the temperature and the deposition potential.To maintain a constant composition the deposition potential must bereduced as the temperature is increased, and vice versa. However it hasbeen found that as the temperature is increased the hysteresis looptends to become less rectangular, the effect becoming very noticeableabove 30 C. At this temperature the corresponding deposition potentialto maintain the required composition is about 890 mv.

Satisfactory films having rectangular hysteresis loops can be producedin the region below 30 C. and above 890 mv., the required combination oftemperature and potential being a matter of experiment. However it hasbeen found preferable to operate in the temperature range 18 to 22 C.(room temperature), in which range satisfactory films are produced atdeposition potentials of between 920 and 950 mv.

(d) The solution concentration must be kept uniform over the cathodesurface. As deposition proceeds, nickel and iron are removed from thesolution in the vicinity of the cathode, resulting in the less denseliquid rising. The

lower end of the cathode thus receives fresher electrolyte than theupper end, and this results in a change in the composition and magneticproperties along the rod. Rotating the cathode on its own axis helps tobreak up this upward flow of liquid over the rod, while the gas bubblesproduce a rapid upward movement in the liquid of a more or less randomkind.

(e) The pH must be maintained in the range 2 to 3.5. Below 2 excessivehydrogen is evolved, while above 3.5 the oxidation of the ferrous ion toferric is quite rapid.

(f) Although satisfactory films have been produced without initiallystriking the cathode surface with nickel as described above, it ispreferred to include this step to ensure uniform deposition. However, inthe deposition of very thin films the step must be omitted to preventaffecting the magnetic properties.

Additional stress-reducing agents, such as sodium saccharin or sodiumnaphthalene trisulphonate can be added to the solution. Their effectbecomes greater with thicker films.

1n the described example the film Was deposited on a copper rod.Satisfactory films have also been deposited on to nickel, iron, brass,beryllium copper, molybdenum, chromium, palladium and silver.

I claim:

1. An electrolyte for the electrodeposition of an approximately /20nickel/iron film on a former consisting essentially of an aqueoussolution of nickel and ferrous sulphamates wherein the nickel ionconcentration is greater than gm./litre, the ratio of nickel ionconcentration to ferrous ion concentration is between 35:1 and 40:1, andthe pH is between 2 and 3.5.

2. A process of electrodepositing an approximately 80/20 nickel/ironfilm on a former comprising introducing nickel and ferrous sulphamatesto an electrolyte cell to form a solution thereof having a nickel ionconcentration greater than 100 gm./l. and a pH of about 2.0- 3.5,extending the former as a cathode Within the solution and subjecting itto an aligning magnetic field, maintaining the ratio of nickel ion toferrous ion concentration at about 35:140:1, the solution temperaturebelow 30 C., the deposition current at a value such that the depositionpotential is greater than 890 mv., and the solution concentrationsubstantially constant over the surface of the cathode.

3. A process according to claim 2 wherein the solution temperature ismaintained at about 1S22 C. and the deposition current is maintained ata value such that the deposition potential is about 920-950 mv.

4. A. process according to claim 2 wherein the cathode surface isinitially struck by passing a deposition current in the solution of sucha value as to deposit a nickel foundation on the cathode surface.

5. A process according to claim 2 wherein the former is cylindrical andis extended substantially vertically in the solution as the cathode, andthe solution concentration is maintained substantially constant over thesurface of the cathode by rotating it on its cylindrical axis andintroducing a stream of gas bubbles into the solution below the cathode.

References Cited in the file of this patent UNITED STATES PATENTS

2. A PROCESS OF ELECTRODEPOSITING AN APPROXIMATELY 80/20 NICKEL/IRONFILM ON A FORMER COMPRISING INTRODUCING NICKEL AND FERROUS SULPHAMATESTO AN ELECTROLYTE CELL TO FORM A SOLUTION THEREOF HAVING A NICKEL IONCONCENTRATION GREATER THAN 100 GM./1. AND A PH OF ABOUT 2.03.5,EXTENDING THE FORMER AS A CATHODE WITHIN THE SOLUTION AND SUBJECTING ITTO AN ALIGNING MAGNETIC FIELD, MAINTAINING THE RATIO OF NICKEL ION TOFERROUS ION CONCENTRATION AT ABOUT 35:1-40:1, THE SOLUTION TEMPERATUREBELOW 30*C., THE DEPOSITION CURRENT AT A VALUE SUCH THAT THE DEPOSITIONPOTENTIAL IS GREATER THAN 890 MV., AND THE SOLUTION CONCENTRATIONSUBSTANTIALLY CONSTANT OVER THE SURFACE OF THE CATHODE.